Wave soldering is a soldering technique widely used in the printed circuit board (PCB) manufacturing industry. This technique uses a solder tank to store molten solder; the molten solder is delivered to a nozzle, from which it is discharged to form a solder wave; a PCB with components inserted is passed over the solder wave by a conveying device and comes into contact with it. The molten solder is adhered on the exposed metal parts (i.e., the parts not protected by a mask) on the circuit board, thus forming reliable mechanical and electrical connections on the circuit board.
FIG. 1 is a schematic diagram of an existing typical wave soldering apparatus. As shown, in the solder tank 101 is stored molten solder (e.g., a solder such as a lead and tin alloy). A motor 102 of a pump device drives a rotation shaft 104 to rotate through a transmission belt 103; the rotation shaft 104 is inserted into the solder tank 101 from the top of the solder tank 101; at the end of the rotation shaft 104 are provided helical blades 105, which, when rotating, drive the molten solder through a conduit 106 to a nozzle 107 to be discharged, thus forming a solder wave. At the same time, a circuit board 109 with components 108 inserted is carried on a conveying belt 110 to pass by the nozzle 107, coming into contact with the solder wave discharged from the nozzle 107 thus to perform soldering.
Since wave soldering is performed through the circuit board coming into contact with the solder wave discharged form the nozzle, the design of the nozzle has much impact on the quality of soldering. FIG. 2 illustrates an existing typical single wave nozzle structure. As shown, the nozzle 107 consists of a front plate 201 and an adjustable rear plate 202; and solder will be discharged from between the two plates and move towards the two sides, forming a flat wave. Such a nozzle design has the defect of being prone to produce the shadow effect. FIG. 3 is a schematic diagram illustrating the production of the shadow effect. As shown in the upper part of FIG. 3, when the circuit board 109 moving in one direction comes into contact with the molten solder moving in the other direction, at the root of the component protruding on the circuit board 109 some air will be entrapped, thus forming bubbles, lowering the soldering quality. Additionally, as shown in the lower part of FIG. 3, the solder cannot fill sufficiently the insert hole on the circuit board 109, leaving some space, which also affects the soldering quality.
FIG. 4 illustrates an existing typical double wave nozzle structure. As shown, the circuit board 109 will pass by two nozzles 401 and 402 when moving on the conveying belt. The first nozzle 401 comprises two side plates 411 and a block piece 412 between the side plates. The solder is discharged from the small gaps formed between the block piece 412 and side plates 411, forming a fast turbulent wave, which is conducive to eliminate the shadow effect and bubbles. The second nozzle 402 comprises a front plate 421 and rear plate 422 distanced farther apart, and the solder is discharged from between the front plate 421 and rear plate 422, forming a flat wave, which can reshaping the solder points. While such a double wave nozzle structure can eliminate the shadow effect, it needs two nozzles with different structures, which usually need to be driven by two pumps, resulting in a complex apparatus, high cost and high energy consumption. Additionally, such a double wave nozzle structure makes the contact area between the solder and the air larger, thus producing more dross. Further, using such a nozzle structure, the high temperature molten solder on the circuit board and components after the turbulent wave soldering will become solidified due to the lower temperature, thus preventing the high temperature molten solder to wick during the flat wave soldering, further degrading the soldering quality. Therefore, in the industry, usually only flat wave soldering is used to solder insert components.
Additionally, as known in the field, when the solder in the solder tank comes into contact with air, tin oxide (SnO and SnO2) will be produced, thus forming dross. During the process of wave soldering, the top surface of the molten solder in the solder tank is exposed in the air, thus dross is formed easily. Especially around the rotation shaft 104, due to the fast air flow, a large amount of dross is formed more easily. The forming of dross causes a great waste of material, and also increases the cleaning cost.
Further, during the process of wave soldering, when the high temperature molten solder comes into contact with the circuit board, a large amount of high temperature waste gas will be produced, causing loss of energy.
It can be seen that there is a need for an improved wave soldering technique which can overcome one or more of the shortcomings of the existing wave soldering techniques in the field.